Fossil “beardogs” shed light on evolution of dogs and their relatives

Large, ferocious-looking animals called beardogs—neither bears nor dogs—roamed the northern hemisphere between about 40 and 5 million years ago. But because so little data on their earliest members are available, their evolutionary relationships to other animals have remained unclear.

In a new paper published in the journal Royal Society Open Science, paleontologists revisited two species of fossil carnivores whose true identities had remained obscure for years, finding them to be some of the earliest and most primitive members of the beardog family.

Artist's reconstruction of an early beardog from Texas, based on fossils of Angelarctocyon australis and Gustafsonia cognita.
@M. Jurik/The Field Museum

First described in 1986, the small Texas fossils—about the size of chihuahuas—were originally assigned to the genus Miacis, a kind of “miscellaneous” category for early carnivores, based primarily on external features.

“It was the best that could be done at the time,” said Jack Tseng, a co-author on the paper, an assistant professor at the University at Buffalo, and a research associate at the Museum, where he was recently a post-doctoral fellow. “It’s a kind of ‘trashbin’ genus, used when the question is, ‘Well, what else could it be?’”

Tseng’s collaborator Susumu Tomiya, a postdoctoral scholar at The Field Museum, stumbled upon one of the specimens in the Chicago museum’s collection (the other specimen is housed at the University of Texas at Austin).

“I thought it looked odd and too advanced for what it had been claimed to be—a more primitive carnivore,” Tomiya said. “It reminded me of some much larger beardogs so I decided to take a closer look.”

The researchers used high-resolution X-ray computed tomography (CT) to create 3D reconstructions of the intracranial anatomy of the fossils. While CT scans of the skulls already existed, the new work resulted in a much more detailed analysis. The reconstructions revealed a “deep embayment”—a bone-enclosed space—in the animals’ ear region, a feature that’s characteristic to beardogs.

Based on the new analysis, the researchers placed the species, which they renamed Gustafsoniacognita and Angelarctocyon australis, at the base of the beardog family tree.

“These are some of the earliest beardogs—they lived 38 to 37 million years ago,” Tomiya said. “By about 15 million years ago, the beardog family had given rise to huge predators bigger than modern lions, but the early members reported in this study were tiny.”

The research also reveals that although beardogs have been known to exist throughout northern continents and Africa, their origins may lie in the southwestern United States.